Electronic documents, such as images of scanned books, can be used in a wide variety of electronic devices, such as mobile phones, desktop, laptop, and tablet computers, and dedicated electronic readers. Oftentimes, there is a desire to restrict distribution of such documents for legal or other reasons. For example, a user who buys an electronic book (eBook) may be granted the right to view the book on a limited number of electronic devices, but not to distribute the book to third parties.
Digital watermarking is a form of digital rights management (DRM) used to prevent unauthorized distribution of an electronic document. This type of watermarking inserts information into the document that can be used to assert ownership of the document. The watermark serves to disincentivize a user from improperly distributing the document by allowing the document to be traced back to the original purchaser. Often the watermark is invisible in the sense that it is not perceived by the user.
There are a variety of known schemes for watermarking digital images. For example, the colors within an image can be subtly altered to encode the ownership information. However, these schemes are often unsuitable for use with electronic documents that contain primarily textual images. The low-bit color space (e.g., black and white text) in such images provides fewer avenues for invisibly encoding information within the image. Moreover, the textual nature of the images also makes it easier for a malicious user to identify and remove the watermark.